E-beam welding eyed for dense nanoscale circuits

TROY, N.Y.  An international consortium of researchers believes its electron-beam method for joining nanotubes could be applied to the construction of ultradense circuits. Rather than add a "glue" material between nanotubes, the electron beam knocks out atoms from between touching nanotubes. The tubes heal the defect by sharing an atom and thereby create a weld between them.

Researchers from Belgium, England, France, Germany, Mexico and the United States cooperated to demonstrate how the welding technique can yield both crossbar and transistor-like three-terminal devices.

"We have not characterized these junctions; they are so small that it will be a real challenge to try doing that. But we have demonstrated that nano-welding is possible, and that's something researchers have long speculated about," said materials science professor Pulickel Ajayan at Rensselaer Polytechnic Institute, here.

Single-walled nanotubes measure only about 1.2 nanometers in diameter  so small that it is almost impossible to manipulate them individually, even with an atomic-force microscope. What's worse, to weld the nanotubes together, they have to be touching each other at precisely the spot where they will be joined, but so far there is no known method for controlling where they touch. Consequently, while welding single-walled nanotubes has long been considered a promising method for creating atomically small semiconductor-like junctions, until now researchers have succeeded in welding only larger, multiwalled tubular carbon fibers. That feat was accomplished by one of the authors of the report on the current experiment.

"We wanted to do this experiment two years ago, but we had a hard time making single-walled nanotubes that were touching in just the right way," said Ajayan.

Nanotubes are hollow tubes formed from the same material as buckyballs: sheets of graphite that consist of hexagonal rings of carbon atoms. At high temperatures and in the right atmosphere, the sheets will curve around to join opposite edges, creating a variety of closed forms such as spheres and tubes. By blowing out one of the carbon atoms from each nanotube right at the point where they touch, researchers had speculated that the opened bonds would spontaneously join so that each tube would share an atom with the other. Such welded junctions, shaped like an X and a T (or a Y), have been proposed as the future's atomically small crossbar switches and transistors, respectively.

"There is a long way to go before we can build anything like a working circuit, but at least we now know that welding single-walled nanotubes is possible," said Ajayan. "We need knowledge like this now, [since] in the future, we are not going to be able to do everything with self-assembly. We will need manual steps, like welding, to build real nanoscale circuits."

Controlled fabrication

Ajayan maintained that welding together single-walled carbon nanotubes could pave the way for controlled fabrication of molecular circuits and nanotube networks. For instance, a matrix of auto-assembling perpendicular nanotubes could be turned into a crossbar switch by using an electron beam to weld the tubes together after they are fabricated. Similarly, atomically small transistor-like three-terminal T and Y junctions could be auto-assembled as needed across a chip, with a final welding step turning them into working transistors.

The group of researchers originally met at the Max-Planck-Institut fur Metallforschung (Stuttgart, Germany). There, they dispersed single-walled nanotubes ultrasonically in ethanol and subsequently deposited them onto holey carbon grids for further observation and processing with a transmission electron microscope (TEM). A 1.25-million-volt accelerating voltage enabled the TEM to generate a 10-amp/centimeter2 energy beam.

Using a heating stage at 800°C, the team observed the real-time actions of the TEM beam using a slow-scan CCD camera. After identifying several crossing points where nanotubes appeared to be touching, the controlled electron beam was set to irradiate the desired junction point. The camera monitored the point to confirm success.

The team claims to have successfully fabricated X, Y and T junctions, as well as some oddball, four-terminal types.

The team was also able to use the beam as a cutting torch, severing a leg from an X crossing to transform it into a T or Y junction. And they report being able to weld small nanotubes to large ones, as will be needed in the future to permit a gate nanotube to control the current in the channel nanotube. But no junction characterization was performed in this experiment.

Unfavorable results

The group also reported some unfavorable results. Many of the irradiated junctions were observed to appear defective in their atomic arrangement, having seven or eight  instead of the usual six  carbon atoms in the ring where atoms are shared between nanotubes. Also, the "disconnected" ends of the rings where they were broken by the electron beam were observed to dangle freely until coaxed into bonding with the adjacent tube by raising the temperature to 800°C. Ajayan speculates that a final step of annealing the whole chip may be required to "solidify" all the welds, but he said the team has yet to try that technique.

The group also wrote a computer simulation of the process that estimated that an electron beam at a temperature of 1,000°C could weld two nanotubes together with heptagonal and octagonal bonds around a complete X junction. Simulations further showed that the bonds so formed would only be 189 electron-volts (eV) less stable than the original hexagon.

An audio recording of reporter R. Colin Johnson's full interview with Pulickel Ajayan can be found online at AmpCast.com/RColinJohnson.